Process for producing high-purity niobium and tantalum

ABSTRACT

A process for producing high-purity niobium and tantalum wherein the oxide of the metal is intimately mixed with carbon, e.g., fine graphite, in an amount such that the oxygen is present in a slight excess beyond the quantity of carbon required to react stoichiometrically with the metal oxide. In the first stage, the mixture is subjected to a high vacuum at the order of 10 4 torr. at a temperature of about 1,800* C. to carry out an initial reduction, the reduced material containing about 500 to 10,000 parts per million (p.p.m.) of oxygen. In an intermediate stage, the partly reduced product is combined with finely divided carbon pyrolytically precipitated from a hydrocarbon in a retort permeable to hydrogen (at elevated temperature), so that the carbon is uniformly distributed over the surface of the partly reduced product. In the second state, the mixture of the partially reduced product and the finely divided pyrolytically precipitated carbon is subjected to a temperature below to 2,000* C. and nevertheless sufficient to effect a final reduction. Preferably the latter temperature is about 1,700* C. The resulting high-purity metal (i.e., tantalum or niobium), may be used in electrolytic capacitors.

United States Patent Restelli Mar. 7, 1972 [54] PROCESS FOR PRODUCINGHIGH- PURITY NIOBIUM AND TANTALUM [72] Inventor: Attilio Restelli,Binningen, Switzerland [73] Assignee: Hermann C. Starck Berlin, Berlin,Germany [22] Filed: June 4,1969

[21] Appl.No.: 830,542

[30] Foreign Application Priority Data June 6, 1968 Switzerland..8380/68 [52] US. Cl ..75/84 [51] Int. Cl. C22b 51/00 [58] Field ofSearch ..75/84, 0.5; 176/91 SP, 67; 252/301.1

[56] References Cited UNITED STATES PATENTS 3,114,629 12/1963 Downing eta1 ..75/84 3,144,328 8/1964 Doty ..75/84 3,231,408 1/1966 Huddle.....l76/89 Re26,294 11/1967 Sowman et a1 ..176/67 Daendliker et al..75/5 BB 3,499,753 3/1970 Daendliker ..75/5 BB Primary ExaminerReubenEpstein Attorney-Karl F. Ross [57} ABSTRACT A process for producinghigh-purity niobium and tantalum wherein the oxide of the metal isintimately mixed with carbon, e.g., fine graphite, in an amount suchthat the oxygen is present in a slight excess beyond the quantity ofcarbon required to react stoichiometrically with the metal oxide. 1n thefirst stage, the mixture is subjected to a high vacuum at the order of10" torr. at a temperature of about 1 ,800 C. to carry out an initialreduction, the reduced material containing about 500 to 10,000 parts permillion (p.p.m.) of oxygen. In an intermediate stage, the partly reducedproduct is combined with finely divided carbon pyrolyticallyprecipitated from a hydrocarbon in a retort permeable to hydrogen (atelevated temperature), so that the carbon is uniformly distributed overthe surface of the partly reduced product. In the second state, themixture of the partially reduced product and the finely dividedpyrolytically precipitated carbon is subjected to a temperature below to2,000 C. and nevertheless sufficient to effect a final reduction.Preferably the latter temperature is about l,700 C. The resultinghigh-purity metal (i.e., tantalum or niobium), may be used inelectrolytic capacitors.

8 Claims, No Drawings PROCESS FOR PRODUCING I-IIGII-PURITY NIOBIUM ANDTANTALUM l. FIELD OF THE INVENTION My present invention relates to aprocess for the production of high-purity metallic tantalum and niobiumand, more particularly, to a process for producing tantalum or niobiummetal low in oxygen and carbon and particularly suitable for use inelectrolytic capacitors.

2. BACKGROUND OF THE INVENTION It has been proposed heretofore toproduce metallic tantalum and/or niobium by reduction of the oxides ofthese metals with the corresponding carbide or with elemental carbon(e.g., in the form of graphite) in high-vacuum furnaces at elevatedtemperatures.

Such processes have, however, the disadvantage that, when carbides arerequired, these carbides must be produced as intermediate products atadditional costs. Whether or not the carbide is used, it has been foundto be difficult using these earlier techniques to obtain an end productboth low in oxygen and low in carbon and capable of being usedsuccessfully as electrolytic-condenser plates.

One' of the problems arising in the prior art systems is that theintermediate product of the high-temperature reaction is ametal/oxygen/carbon system which is created during the initial reductionstage. When the precise quantity of carbon necessary to reactstoichiometrically with all of the oxygen is used, it is found to beimpossible, in practice, to maintain such precise stoichiometrythroughout the entire charge.

This difficulty arises from the fact that the mobility of oxygen in theoxide/oxygen/carbon system is relatively high whereas that of carbon isrelatively low. Limited, although hardly avoidable, temperaturedifferentials within the charge result in a concentration of oxygen atcertain localities therewithin while carbon-rich locations are foundelsewhere. In fact, these localized carbon concentrations can notadequately be brought into intimate relationship with oxygenrich areaseven during prolonged heating after mixture; also any interactionbetween carbon and oxygen ceases while the charge containsproportionately large quantities of both. The residual oxygen appears tobe present, to a certain extent at least, in the form of suboxides whichcan be volatilized by an increase in temperature at the end of thereduction stage.

The volatilization step has the additional disadvantage that thevaporized metal suboxides can not be recovered and result in a loss ofthe starting material. Moreover, there is a tendency, at the elevatedtemperatures which must be used to vaporize the suboxides, for the metalsuboxides to react with the material forming the reaction vessel orcrucible and result in a sloughing of crucible material into thereacting mass. The

latter difficulty introduces a further impurity, which may render therecovered metal unsuitable for use in electrolytic capacitors.

Still further, since the temperature required for volatilizing thesuperfluous oxygen in the form of suboxides of the metal generally rangeabove 2,000 C., there occurs a sintering or fusing of the reactantmaterial to itself and to the reaction vessel; this makes more difficultthe removal of the charge from the reaction vessel. In addition thefused or sintered mass must be broken up or comminuted, thereby givingrise to a comminution step and a formation of fresh surfaces subject toatmospheric oxidation. Such oxidation detrimentally influences theability to use the metal as sintered anodes in electrolytic capacitorsof the type mentioned earlier.

3. OBJECTS OF THE INVENTION It is, therefore, the principal object ofthe present invention to provide an improved process for the productionof highpurity tantalum and niobium which is particularly suited for useas a sintered plate in an electrolytic capacitor and which avoids thedisadvantages of the prior art processes mentioned earlier.

Another object of this invention is to provide a process for producinghigh-purity tantalum and niobium which yields a product low in oxygenand carbon and which does not require mechanical comminution with thedisadvantages thus entailed.

Yet another object of this invention is to provide a process for theproduction of high-purity tantalum and niobium which can be operated attemperatures below 2,000 C., thereby avoiding sintering of the mass,consequent difficulty of removing the mass from the reaction vessel, andthe necessity of comminuting the mass.

It is still further an object of the instant invention to provide aprocess for the production of tantalum and niobium, from the oxidesthereof, which reduces loss of the metal in the form of its suboxides,precludes contamination of the metal with material derived from thereaction vessel, and results in a product with a lower oxygen contentthan has been attainable heretofore.

Yet a further object of the instant invention is to provide a processfor the production of high-purity niobium and tantalum which extendsprinciples set forth in the commonly assigned copending applicationsSer. No. 609,00l filed 13 Jan. 1967 (now US. Pat. No. 3,499,753) andSer. No. 718,929 (now abandoned) and filed 4 Apr. 1968 by myself andGustav Daedliker.

4. SUMMARY OF THE INVENTION These objects and others which will becomeapparent hereinafter are attained, in accordance with the presentinvention, in a two-stage process for reducing niobium and tantalumoxides, wherein, in the first stage, the oxide of the metal to berecovered in a low-oxygen, low-carbon condition suitable for use inelectrolytic capacitors, is intimately mixed with carbon, e.g., in theform of finely divided graphite, in an amount just less than thequantity required to stoichiometrically react all of the oxygen of thematerial to form carbon monoxide; the first-stage reduction process iscarried out in a vacuum furnace under a negative pressure of the orderof 10 torr and at a temperature below 2,000 C. to yield a productcontaining about 500 to 10,000 parts per million (p.p.m.) of oxygen. Anoxygen excess of at most 1 percent thus remains in the firststagereduction produce. In a second step of the instant process, intermediateto two reduction stages, I deposit upon the surfaces of the reducedproduct of the first stage a finely divided elemental carbon obtainedfrom the pyrolytic decomposition of a hydrocarbon, especially aparaffinic alkane having a carbon number ranging between one and eight.

In this intermediate step, the precipitated pyrolytic carbon isintimately mixed with the reduced product of the initial stage,whereupon the mixture of finely divided carbon and partially reducedoxide, wherein the carbon content now is stoichiometrically equal to thequantity necessary to react all of the oxygen remaining, is reacted at atemperature below to 2000 C. in the second reduction stage and atreduced pressure to yield a final product which may be used inelectrolytic capacitors as will be apparent hereinafter.

By pyrolytic precipitation of carbon in finely divided form uniformlyover the surfaces of the prereduced or partially reduced product of thefirst stage and following this precipitation by an intimate mixture ofthe partially reduced oxide and precipitated carbon, 1 an able tocompletely eliminate the tendency toward the formation of segregationzones containing carbon-rich or oxygen-rich materials which areincapable of interacting. Moreover the volatilization stage can becompletely eliminated inasmuch as no substantial proportion of suboxideremains.

According to an important feature of this invention, the precipitationof the finely divided carbon film is effected by pyrolysis at or abovethe pyrolyzing temperature of the gaseous hydrocarbon and especially ahydrocarbon of the paraffin series on a heated metal surface accordingto the formula:

t: H hC (N-l-l H wherein n IS an integer ranging from one to eight.

The reaction vessel. according to the present invention. is a materialwhich. at elevated temperatures leg. 700 to l.000 t1). is permeable tohydrogen. for example. a nickel-chromi- .llll-llOn alloy. By evacuatingthe furnace containing the sealed retort. I initially am able to effecta hydrogen diffusion from the interior of the vessel and thereby controlthe amount of precipitated carbon which appears to from in reproducibleratios upon the wall of the vessel and upon the surfaces of the charge.The evacuated hydrogen is burned off. Best results have been found withpyrolysis temperatures ranging between and l.000 C.

lurprisingly. the intermediate stage wherein a finely divided .iarbonfilm is precipitated on the surface of the partially reduced metal.permits the second reaction stage to be carried out at temperatures wellbelow l.000 C. preferably at about llS purity and grain structure. mostsuitable for high quality i imter anodes of niobium and tantalum inhigh-capacity elecirolytic condensers. Moreover. in the course of theprocess. the concentrations of nonrefractory impurity metals are reducedto less than five p.p.m.

l. SPECIFIC EXAMPLES The following examples are illustrative of thepresent process.

EXAMPLE] Thirty Kilograms (kg) of tantalum pentoxide powder intimatelymixed with a fine annealed graphite a 99.6 percent purity in an amountof 4.040 grams lg.) and pressed into tablets weighing two grams ig.)each. The tablets are uniformly heated in a highvacuum furnace to atemperature of l.800 C. to react the graphite with the oxide and formcarlhonmonoxide. The reaction temperature is maintained as long .iscarbonmonoxide is evolved and the vacuum brought to a iubatmospheric ornegative pressure value of about 1 to 5-l0 orr.

The resulting partially reduced tantalum. constituting the first-stageproduct. has an average oxygen content of 1.720 ppm. and a carboncontent of 65 p.p.m.

Of this first-stage reduction product. i .2 kg. is charged .into aretort composed of the nickel alloy known as lnconel "r00. {A suitablelnconel alloy may consist of 77-105 percent iy weight nickel. l4ilpercent by weight chromium. 0.21-0.05 percent by weight copper. 0.5:1percent by weight iron. 0.5103 percent by weight manganese. 10.75percent by weight silicon. 0.08 to 0.2 percent by weight carbon and ispermeable by hydrogen at elevated temperatures.)

The retort is placed bodily in the high-vacuum furnace and heated to atemperature of 900 C. whereupon 38.000 torr X liter (corrected to 0 C.)of methane is added in portions. This .sorresponds approximately to 26.7g. of carbon. The amount if carbon taken up is determined by thecumulative pressure tlifference. As the methane contacts the interior ofthe retort and the prereduced metal therein. it pyrolyzes and deposits:1 iinely divided carbon film over these surfaces. The methane pyrolysisresults in a gradual reduction of the pressure from the starting level.During pyrolysis. hydrogen lS released and iiiffuses through the wall ofthe retort. the retort being hermetically sealed except for diffusionthrough its walls.

The retort is then discharged and the mixture of pyrolyticallydischarged carbon thoroughly mixed with the tantalum pellets. Theaverage carbon content is determined as 690 p.p.m. and 12.0 g. of carbonare calculated as taken up by the metal.

he resulting mixture is then subjected to second-stage "eductionaccording to the present invention as described in connection with thefirst reduction stage under vacuum at a temperature of l.850 C. at whichthe mass held until the pressure within the high-vacuum furnace isreduced to 2Xl0 torr. The resulting tantalum has an oxygen content of530 p.p.m. and a carbon content of p.p.m.

The tantalum is particularly suited for use in electrolytic capacitorsand can be formed into plates as described in US. Pat. No. 3.430.108 orthe above-identified application Ser. No. 718.929.

EXAMPLE ll As described in Example l. 30 kg. of tantalum pentoxide isntimately mixed with 4.040 g. of graphite powder. pressed .nto tabletsand sintered in vacuo. The resulting first-stage "ECIUCUOH product hasan oxygen content of 2.020 p.p.m. and .1 carbon content of 40 p.p.m.

The intermediate carbon correction is carried out with 23 xg. of butanewhereby l [.700 torr X liters at 800C. is reacted o pyrolyticallyprecipitate the carbon film.

The carbon-coated tablets are thoroughly mixed and inalyzed and theaverage carbon content is found to be about 900 p.p.m.

The second-stage reduction is carried out by sintering in iacuum (seeExample I) at a temperature of l.850 C. to obam a nigh-purity tantalumproduct with only [30 p.p.m. of ixvgen.

EXAMPLE Ill Eighteen g. of tantalum tablets recovered from a first-stagefiaCIlOl'l of tantalum pentoxide and graphite powder as .iescribed inthe previous Examples. and having an oxygen content of 4.420 p.p.m. anda carbon content of 20 p.p.m. is :reated in a retort of lnconel 600 at900 C. with butane in an mount of 25.000 torr X liters. The averagecarbon content of the thoroughly mixed mass. after deposition of thefinely dirided pyrolytic carbon. is found to be about 2.650 p.p.m. Themass is sintered in vacuum at a temperature of l.850 C. as in Example lto yield tantalum containing 250 p.p.m. oxygen and tlO-QO p.p.m. carbon.The tantalum is hydrogenated and TllllCd to a fine powder (seeapplications Ser. No. 609.00l and No. 718.929) by conventionaltechniques and is thereafter iubiected to dehydrogenation to obtain ametal powder with in average particle size of seven microns. This powderconraining 1.650 p.p.m. oxygen. 50 p.p.m. nitrogen and p.p.m. carbon ischaracterized by a low content of metallic impurities. The total amountof nickel. chromium. manganese. magriesium. aluminum. silicon. calcium.copper, titanium. zirconium and iron is less than five p.p.m.

This powder is formed into sintered anodes for condensers as describedgenerally in the aforementioned application and U.S. patent. Moreparticularly. 1.98 g. of the powder is pressed into an anode. with adiameter of 6.7 mm. and a iDBClfiC gravity of 8.4 g/cm: and sintered for30 minutes at 950 C. in high vacuum. The resulting anode had anelectrical capacity of about 6.240;; FV. The breakdown voltage of theelectrode in 0.1 percent H PO is determined to be in excess of 250volts. The tantalum powders of Example I and ll yield similar results.

What is claimed is:

A process for producing high-purity niobium and tantalum from acorresponding metal oxide. comprising the steps .l. reducing the metaloxide by intimately mixing it with eleiiiental carbon and subjecting theresulting mixture to an cievated temperature in vacuo to produce aprereduced iroduct with an oxygen content of about 500 to 10.000 p.p.m..the elemental carbon mixed with the metal oxide vieing present in anamount less than the quantity "BQUII'Cd to stoichiometrically react allof the oxygen of he oxide to form carbon monoxide and said oxygen isiresent in an excess of at most one percent beyond thatstoichiometrically calculated to react with the elemental carbon;

b. heating the prereduced produce and contacting same with a gaseoushydrocarbon to pyrolytically precipitate elemental carbon on theprereduced product to form an intimate combination of the pyrolyticallyprecipitated carbon and the prereduced product; and

c. subjecting the combination of pyrolytically precipitated carbon andthe prereduced product to a second reducing stage at a temperature belowabout 2,000" C. but sufficient to react the pyrolytically precipitatedcarbon with oxygen retained in the prereduced product.

2. The process defined in claim 1 wherein said prereduced product isreacted with said hydrocarbon at a temperature between 700 and 1,000 C.

3. The process defined in claim 2 wherein said prereduced product isreacted with said hydrocarbon in a sealed reaction vessel having ahydrogen-permeable wall at a temperature of 700 to l,000 C.

4. The process defined in claim 3 wherein said reaction vessel iscomposed of nickel-chromium-iron alloy.

5. The process defined in claim 3 wherein said hydrocarbon is aparaffinic alkane of the general formula C,,H where n is an integerranging between l and 8.

6. The process defined in claim 5 wherein steps (a) and (c) are eachcarried out at a temperature below 2,000" C.

7 The process defined in claim 6 wherein steps (a) and (c) are eachcarried out at a pressure ofthe order of 10" torr.

8. The process defined in claim 7 wherein step (c) is carried out at atemperature of about 1,700 C.

2. The process defined in claim 1 wherein said prereduced product isreacted with said hydrocarbon at a temperature between 700* and 1,000*C.
 3. The process defined in claim 2 wherein said prereduced product isreacted with said hydrocarbon in a sealed reaction vessel having ahydrogen-permeable wall at a temperature of 700* to 1,000* C.
 4. Theprocess defined in claim 3 wherein said reaction vessel is composed ofnickel-chromium-iron alloy.
 5. The process defined in claim 3 whereinsaid hydrocarbon is a paraffinic alkane of the general formula CnH(2n 2)where n is an integer ranging between 1 and
 8. 6. The process defined inclaim 5 wherein steps (a) and (c) are each carried out at a temperaturebelow 2,000* C. 7 The process defined in claim 6 wherein steps (a) and(c) are each carried out at a pressure of the order of 10 4 torr.
 8. Theprocess defined in claim 7 wherein step (c) is carried out at atemperature of about 1,700* C.